FirSt-order equational logic conSiStS of quantifier-free termS of ordinary firSt-order logic, with equality aS the only predicateSymbol. The model theory of thiS logic waS developed into UniverSal algebra by Birkhoff et al. [Birkhoff, Gratzer, Cohn]. It waS later made into a branch of category theory by Lawvere ("algebraic theorieS"). (1995-02-21)